Photoammonification Of Typical Low-molecular-weight Dissolved Organic Nitrogen In Surface Water System

Phototransformation of dissolved organic nitrogen（DON）to dissolved inorganic nitrogen（DIN）is an important part of nitrogen cycle in surface aquatic system.This study measured photoammonification rates of typical low-molecular-weight（LMW）DON（e.g.,amino acids,nucleotides and urea）under different conditions to elucidate the potential effects of DON structures,light sources and reactive species.Results showed that under simulated sunlight,amino acids with aromatic moieties produced different amount of NH₄⁺.Tyrosine and tryptophan exhibited the highest ammonification rates,up to 50%yield within 6 hours,whereas negligible photoammonification was observed for urea,adenine and cytosine.Hydroxyl radical can promote ammonia yields of all the target amino acids,especially played a crucial role on methionine and tryptophan photoammonification.Triplet excited CDOM contributed variedly to the five amino acids photoammonification but imposed limitation on the NH₄⁺yield of tyrosine.Singlet oxygen only had a positive impact on tryptophan ammonification.Carbonate radical accelerated ammonia generation of histidine whereas it had no impact on methionine and tryptophan and had an inhibited impact on phenylalanine and tyrosine.Under natural sunlight,tyrosine and tryptophan produced remarkable NH₄⁺by phototransformation in both buffer solution and natural water samples,while histidine and methionine in natural water samples could also form significant NH₄⁺.The different impacts of reactive species illustrated that diverse water components affect the photoammonification of LMW DON differently.When discussing the influence of various light sources on the photoammonification of LMW DON,the results showed that the most effective range of wave length was UV light while negligible contribution from visible light was found.Also,the structures of DON contributed to photoammonification process.Based on density functional theory calculation,significant negative linear correlation between photoammonification rates and E_LUMO-E_HOMO of amino acids was found.The study above mainly introduced the complexity of DON photoammonification in surface water system,indicating that the significant role of photoammonification of LMW DON in surface water system should be highlighted in nitrogen biogeochemical cycle,and the assessment of water quality and ecology environment.